'Christian' isn't broken down at all in the EW Census (people weren't asked to specify), but religions included under 'Other religions' are listed by name in the 'detailed' religion table (KS210EW). If there's a comparable detailed table for the NI results, you could see if the Christian-related religions are included under 'Other religions' for EW or not. You would then need to assume that people just ticked Christian for those religions, so not sure it will give you a definitive answer, but might help...

At a minimum you'll need to be very careful about this, but my view is that you really can't match up religion definitions between the different UK countries at all because the questions are asked so differently.

I'm more familiar with the EW / Scotland comparison than the Northern Irish one, but I think there will be similar issues for NI. Here's one of the intra-GB issues.

In 2001 (nothing significant has changed since 2001 in the ways the questions are asked) there were far more people classified as having no religion in Scotland (27.5%) than there were in England and Wales (14.8%). This doesn't reflect any underlying difference in distribution of religious beliefs, but happened mainly because the Scottish questionnaire demanded that Christians specify what kind of Christian they were, while the EW questionnaire did not. So people who regarded themselves as being Christian in a vague sort of way but had no affiliation to any church could easily respond 'Christian' in England or Wales, but the Scottish request for specification of a kind of Christianity might push the same person to respond 'no religion' were they to fill in a Scottish form.

The net result is that 'no religion' in Scotland isn't comparable to 'no religion' in England and Wales. It would have been nice to have had a bit more harmonisation of the religion question in 2011, but it didn't happen, so any comparison of religion across countries might be very dubious.

The difference between the Northern Irish question and the E&W question looks much greater than the difference between the Scottish question and the E&W question.

"... the Scottish questionnaire demanded that Christians specify what kind of Christian they were, while the EW questionnaire did not."

This may well be part of the reason for the high proportion of people in Scotland with "no religion". However, another important factor is likely to be that, in 2001, the Scottish census had two religion questions - current religion, and religion of upbringing.

So, people with a loose affiliation (to any religion - not just Christianity) may have used the opportunity to give different answers for these questions. And whilst, as you noted, 27.5% of Scots stated that they currently had no religion, only 17.5% stated that they were not brought up in any religion.

For 2011, Scotland switched to a single question only ("What religion, religious denomination or body do you belong to?"). NI however retained two questions.

At a minimum you'll need to be very careful about this, but my view is that you really can't match up religion definitions between the different UK countries at all because the questions are asked so differently.

I'm more familiar with the EW / Scotland comparison than the Northern Irish one, but I think there will be similar issues for NI. Here's one of the intra-GB issues.

In 2001 (nothing significant has changed since 2001 in the ways the questions are asked) there were far more people classified as having no religion in Scotland (27.5%) than there were in England and Wales (14.8%). This doesn't reflect any underlying difference in distribution of religious beliefs, but happened mainly because the Scottish questionnaire demanded that Christians specify what kind of Christian they were, while the EW questionnaire did not. So people who regarded themselves as being Christian in a vague sort of way but had no affiliation to any church could easily respond 'Christian' in England or Wales, but the Scottish request for specification of a kind of Christianity might push the same person to respond 'no religion' were they to fill in a Scottish form.

The net result is that 'no religion' in Scotland isn't comparable to 'no religion' in England and Wales. It would have been nice to have had a bit more harmonisation of the religion question in 2011, but it didn't happen, so any comparison of religion across countries might be very dubious.

The difference between the Northern Irish question and the E&W question looks much greater than the difference between the Scottish question and the E&W question.